Apparatus for analyzing multi-layer thin film stacks on semiconductors

ABSTRACT

An optical measurement system is disclosed for evaluating samples with multi-layer thin film stacks. The optical measurement system includes a reference ellipsometer and one or more non-contact optical measurement devices. The reference ellipsometer is used to calibrate the other optical measurement devices. Once calibration is completed, the system can be used to analyze multi-layer thin film stacks. In particular, the reference ellipsometer provides a measurement which can be used to determine the total optical thickness of the stack. Using that information coupled with the measurements made by the other optical measurement devices, more accurate information about individual layers can be obtained.

FIELD OF THE INVENTION

[0001] The present invention relates to optical analyzers, and moreparticularly to an optical measurement system having a stable singlewavelength ellipsometer and a broadband spectroscopic measurement moduleto accurately characterize multi-layer thin film stacks.

BACKGROUND OF THE INVENTION

[0002] There is considerable interest in developing systems foraccurately measuring the thickness and/or composition of multi-layerthin films. The need is particularly acute in the semiconductormanufacturing industry where the thickness of these thin film oxidelayers on semiconductor substrates is measured. To be useful, themeasurement system must be able to determine the thickness and/orcomposition of films with a high degree of accuracy. The preferredmeasurement systems rely on non-contact, optical measurement techniques,which can be performed during the semiconductor manufacturing processwithout damaging the wafer sample. Such optical measurement techniquesinclude directing a probe beam to the sample, and measuring one or moreoptical parameters of the reflected probe beam.

[0003] In order to increase measurement accuracy and to gain additionalinformation about the target sample, multiple optical measuring devicesare often incorporated into a single composite optical measurementsystem. For example, the present assignee has marketed a product calledOPTI-PROBE, which incorporates several optical measurement devices,including a Beam Profile Reflectometer (BPR), a Beam ProfileEllipsometer (BPE), and a Broadband Reflective Spectrometer (BRS). Eachof these devices measures parameters of optical beams reflected by thetarget sample. The BPR and BPE devices utilize technology described inU.S. Pat. Nos. 4,999,014 and 5,181,080 respectively, which areincorporated herein by reference.

[0004] The composite measurement system mentioned above combines themeasured results of each of the measurement devices to precisely derivethe thickness and composition of the thin film and substrate of thetarget sample. However, the accuracy of the measured results dependsupon precise initial and periodic calibration of the measurement devicesin the optical measurement system. Further, recently developedmeasurement devices have increased sensitivity to more accuratelymeasure thinner films and provide additional information about film andsubstrate composition. These newer systems require very accurate initialcalibration. Further, heat, contamination, optical damage, alignment,etc., that can occur over time in optical measurement devices, affectthe accuracy of the measured results. Therefore, periodic calibration isnecessary to maintain the accuracy of the composite optical measurementsystem.

[0005] It is known to calibrate optical measurement devices by providinga reference sample having a known substrate, with a thin film thereonhaving a known composition and thickness. The reference sample is placedin the measurement system, and each optical measurement device measuresthe optical parameters of the reference sample, and is calibrated usingthe results from the reference sample and comparing them to the knownfilm thickness and composition. A common reference sample is a “nativeoxide” reference sample, which is a silicon substrate with an oxidelayer formed thereon having a known thickness (about 20 angstroms).After fabrication, the reference sample is kept in a non-oxygenenvironment to minimize any further oxidation and contamination thatchanges the thickness of the reference sample film away from the knownthickness, and thus reduces the effectiveness of the reference samplefor accurate calibration. The same reference sample can be reused toperiodically calibrate the measurement system. However, if and when theamount of oxidation or contamination of the reference sample changes thefilm thickness significantly from the known thickness, the referencesample must be discarded.

[0006] For many optical measurement devices, reference samples withknown thicknesses have been effective for system calibration. Oxidationand contamination that routinely occurs over time with reference samplesis tolerable because the film thickness change resulting from theoxidation/contamination is relatively insignificant compared to theoverall thickness of the film (around 100 angstroms). However, newultra-sensitive optical measurement systems have been recently developedthat can measure film layers with thicknesses less than 10 angstroms.These systems require reference samples having film thicknesses on theorder of 20 angstroms for accurate calibration. For such thin filmreference samples, however, the changes in film layer thicknessresulting from even minimal oxidation or contamination are significantcompared to the overall “known” film layer thickness, and result insignificant calibration error. Therefore, it is extremely difficult, ifnot impossible, to provide a native oxide reference sample with a knownthickness that is stable enough over time to be used for periodiccalibration of ultra-sensitive optical measurement systems.

[0007] There is a need for a calibration method for ultra-sensitiveoptical measurement devices that can utilize a reference sample thatdoes not have a stable or known film thickness.

[0008] There is also a need in the industry to improve the accuracy ofthese type of measuring systems to permit characterization of sampleshaving multiple thin film layers formed thereon. More particularly, inthe semiconductor industry, semiconductor material substrates are nowbeing fabricated with multiple thin film layers. Each film layer can beformed from a different material. Common layer materials include oxides,nitrides, polysilicon, titanium and titanium-nitride.

[0009] Attempts to characterize samples having multiple thin layers withconventional techniques is difficult since each layer has a differentthickness and different optical characteristics. The best approachesfound to date to characterize such complex stacks is to utilize multiplemeasurement techniques which generate independent data that can beanalyzed by a processor. Devices now exist which are capable of makingboth ellipsometric (phase) and spectrophotometric (magnitude)measurements and integrating the results in a microprocessor. Theellipsometers in these devices can include multiple wavelength andmultiple angle of incidence measurements. Similarly, thespectrophotometers in some of these devices can be arranged to makemeasurements at multiple angles of incidence.

[0010] While these systems have had reasonable success, further accuracyin analyzing the characteristics of individual layers in a multi-layerstack is always desirable. The subject system, which includes awavelength stable calibration ellipsometer can be modified to improvethe characterization of individual layers of multi-layer thin filmstack.

SUMMARY OF THE INVENTION

[0011] The present invention is a thin film optical measurement systemwith a wavelength stable ellipsometer that can be used for calibrationand to enhance the characterization of multi-layer thin film stacks.When used for calibration purposes, the stable wavelength ellipsometerfunctions to precisely determine the thickness of a film on a referencesample. The measured results from the calibration ellipsometer are usedto calibrate other optical measurement devices in the thin film opticalmeasurement system. By not having to supply a reference sample with apredetermined known film thickness, a reference sample having a filmwith a known composition can be repeatedly used to calibrateultra-sensitive optical measurement devices, even if oxidation orcontamination of the reference sample changes the thickness of the filmover time.

[0012] The calibration reference ellipsometer uses a reference samplethat has at least a partially known composition to calibrate at leastone other non-contact optical measurement device. The referenceellipsometer includes a light generator that generates aquasi-monochromatic beam of light having a known wavelength and a knownpolarization for interacting with the reference sample. The beam isdirected at a non-normal angle of incidence relative to the referencesample to interact with the reference sample. An analyzer createsinterference between S and P polarized components in the light beamafter the light beam has interacted with reference sample. A detectormeasures the intensity of the light after the beam has passed throughthe analyzer. A processor determines the polarization state of the lightbeam entering the analyzer from the intensity measured by the detector.The processor then determines optical properties of the reference samplebased upon the determined polarization state, the known wavelength oflight from the light generator and the at least partially knowncomposition of the reference sample. The processor operates at least oneother non-contact optical measurement device that measures an opticalparameter of the reference sample. The processor calibrates the otheroptical measurement device by comparing the measured optical parameterfrom the other optical measurement device to the determined opticalproperty from the reference ellipsometer.

[0013] The reference ellipsometer has the further benefit in that it canbe used to very accurately measure the overall optical thickness of anunknown multi-layer stack on a substrate. In this context, the termtotal optical thickness refers to the effective thickness of the stackwhich corresponds to a single uniform layer with uniform opticalparameters (i.e. n and k). A stable wavelength ellipsometer is anexcellent tool for determining the total optical thickness of a layer ora stack having a thicknesses less than 500 angstroms and is the besttool for stacks having a thickness of 200 angstroms or less.

[0014] The reference ellipsometer, which provides only a singlewavelength, single angle of incidence output, is not suitable foranalyzing the individual layers in a stack. Such analysis requiresadditional measurements typically from spectroscopic tools such asspectrophotometers and spectroscopic ellipsometers. However, the lattertools alone have difficulty producing sufficient information toaccurately characterize the stack.

[0015] In accordance with the subject invention, the output from thewavelength stable ellipsometer is used by the processor to determine theoverall optical thickness of the multi-layer stack. This information isused by the processor to reduce the uncertainty of the analysis based onthe spectroscopic measurements. By taking a number of measurements atdifferent wavelengths with one or more different techniques, veryaccurate information about layer composition and thickness can bedetermined.

[0016] Other aspects and features of the present invention will becomeapparent by a review of the specification, claims and appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a plan view of a composite optical measurement systemwith the calibration ellipsometer of the present invention.

[0018]FIG. 2 is a side cross-sectional view of the reflective lens usedwith the present invention.

[0019]FIG. 3 is a plan view of an alternate embodiment of the lightsource for the calibration ellipsometer of the present invention.

[0020]FIG. 4 is a plan view of the composite optical measurement systemwith multiple compensators in the calibration ellipsometer of thepresent invention.

[0021]FIG. 5 is an illustration of a multi-layer stack on a sample.

[0022]FIG. 6 is a flow chart illustrating the steps which can be carriedout to characterize individual layers of a multi-layer stack usingmeasurements from both a stable wavelength ellipsometer and amulti-wavelength measurement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The present invention is a composite thin film opticalmeasurement system 1 having a wavelength stable reference ellipsometer 2that is used, in conjunction with a reference sample 4 having asubstrate 6 and thin film 8 with known compositions, to calibratenon-contact optical measurement devices contained in the composite thinfilm optical measurement system 1.

[0024]FIG. 1 illustrates the composite optical measurement system 1 thathas been developed by the present assignees, which includes fivedifferent non-contact optical measurement devices and the referenceellipsometer 2 of the present invention.

[0025] Composite optical measurement system 1 includes a Beam ProfileEllipsometer (BPE) 10, a Beam Profile Reflectometer (BPR) 12, aBroadband Reflective Spectrometer (BRS) 14, a Deep Ultra VioletReflective Spectrometer (DUV) 16, and a Broadband SpectroscopicEllipsometer (BSE) 18. These five optical measurement devices utilize asfew as two optical sources: laser 20 and white light source 22. Laser 20generates a probe beam 24, and white light source 22 generates probebeam 26 (which is collimated by lens 28 and directed along the same pathas probe beam 24 by mirror 29). Laser 20 ideally is a solid state laserdiode from Toshiba Corp. which emits a linearly polarized 3 mW beam at673 nm. White light source 22 is ideally a deuterium-tungsten lamp thatproduces a 200 mW polychromatic beam that covers a spectrum of 200 nm to800 nm. The probe beams 24/26 are reflected by mirror 30, and passthrough mirror 42 to sample 4.

[0026] The probe beams 24/26 are focused onto the surface of the samplewith a lens 32 or lens 33. In the preferred embodiment, two lenses 32/33are mounted in a turret (not shown) and are alternatively movable intothe path of probe beams 24/26. Lens 32 is a spherical, microscopeobjective lens with a high numerical aperture (on the order of 0.90 NA)to create a large spread of angles of incidence with respect to thesample surface, and to create a spot size of about one micron indiameter. Lens 33 is illustrated in FIG. 2, and is a reflective lenshaving a lower numerical aperture (on the order of 0.4 NA) and capableof focusing deep UV light to a spot size of about 10-15 microns.

[0027] Beam profile ellipsometry (BPE) is discussed in U.S. Pat. No.5,181,080, issued Jan. 19, 1993, which is commonly owned by the presentassignee and is incorporated herein by reference. BPE 10 includes aquarter wave plate 34, polarizer 36, lens 38 and a quad detector 40. Inoperation, linearly polarized probe beam 24 is focused onto sample 4 bylens 32. Light reflected from the sample surface passes up through lens32, through mirrors 42, 30 and 44, and directed into BPE 10 by mirror46. The position of the rays within the reflected probe beam correspondto specific angles of incidence with respect to the sample's surface.Quarter-wave plate 34 retards the phase of one of the polarizationstates of the beam by 90 degrees. Linear polarizer 36 causes the twopolarization states of the beam to interfere with each other. Formaximum signal, the axis of the polarizer 36 should be oriented at anangle of 45 degrees with respect to the fast and slow axis of thequarter-wave plate 34. Detector 40 is a quad-cell detector with fourradially disposed quadrants that each intercept one quarter of the probebeam and generate a separate output signal proportional to the power ofthe portion of the probe beam striking that quadrant. The output signalsfrom each quadrant are sent to a processor 48. As discussed in the U.S.Pat. No. 5,181,080 patent, by monitoring the change in the polarizationstate of the beam, ellipsometric information, such as ψ and Δ, can bedetermined. To determine this information, the processor 48 takes thedifference between the sums of the output signals of diametricallyopposed quadrants, a value which varies linearly with film thickness forvery thin films.

[0028] Beam profile reflectometry (BPR) is discussed in U.S. Pat. No.4,999,014, issued on Mar. 12, 1991, which is commonly owned by thepresent assignee and is incorporated herein by reference. BPR 12includes a lens 50, beam splitter 52 and two linear detector arrays 54and 56 to measure the reflectance of the sample. In operation, linearlypolarized probe beam 24 is focused onto sample 4 by lens 32, withvarious rays within the beam striking the sample surface at a range ofangles of incidence. Light reflected from the sample surface passes upthrough lens 32, through mirrors 42 and 30, and directed into BPR 12 bymirror 44. The position of the rays within the reflected probe beamcorrespond to specific angles of incidence with respect to the sample'ssurface. Lens 50 spatially spreads the beam two-dimensionally. Beamsplitter 52 separates the S and P components of the beam, and detectorarrays 54 and 56 are oriented orthogonal to each other to isolateinformation about S and P polarized light. The higher angles ofincidence rays will fall closer to the opposed ends of the arrays. Theoutput from each element in the diode arrays will correspond todifferent angles of incidence. Detector arrays 54/56 measure theintensity across the reflected probe beam as a function of the angle ofincidence with respect to the sample surface. The processor 48 receivesthe output of the detector arrays 54/56, and derives the thickness andrefractive index of the thin film layer 8 based on these angulardependent intensity measurements by utilizing various types of modelingalgorithms. Optimization routines which use iterative processes such asleast square fitting routines are typically employed. One example ofthis type of optimization routine is described in “MultiparameterMeasurements of Thin Films Using Beam-Profile Reflectivity,” Fanton, et.al., Journal of Applied Physics, Vol. 73, No. 11, p.7035, 1993. Anotherexample appears in “Simultaneous Measurement of Six Layers in a Siliconon Insulator Film Stack Using Spectrophotometry and Beam ProfileReflectometry, ” Leng, et. al., Journal of Applied Physics, Vol. 81, No.8, page 3570, 1997.

[0029] Broadband reflective spectrometer (BRS) 14 simultaneously probesthe sample 4 with multiple wavelengths of light. BRS 14 uses lens 32 andincludes a broadband spectrometer 58 which can be of any type commonlyknown and used in the prior art. The spectrometer 58 shown in FIG. 1includes a lens 60, aperture 62, dispersive element 64 and detectorarray 66. During operation, probe beam 26 from white light source 22 isfocused onto sample 4 by lens 32. Light reflected from the surface ofthe sample passes up through lens 32, and is directed by mirror 42(through mirror 84) to spectrometer 58. The lens 60 focuses the probebeam through aperture 62, which defines a spot in the field of view onthe sample surface to analyze. Dispersive element 64, such as adiffraction grating, prism or holographic plate, angularly disperses thebeam as a function of wavelength to individual detector elementscontained in the detector array 66. The different detector elementsmeasure the optical intensities of the different wavelengths of lightcontained in the probe beam, preferably simultaneously. Alternately,detector 66 can be a CCD camera, or a photomultiplier with suitablydispersive or otherwise wavelength selective optics. It should be notedthat a monochrometer could be used to measure the different wavelengthsserially (one wavelength at a time) using a single detector element.Further, dispersive element 64 can also be configured to disperse thelight as a function of wavelength in one direction, and as a function ofthe angle of incidence with respect to the sample surface in anorthogonal direction, so that simultaneous measurements as a function ofboth wavelength and angle of incidence are possible. Processor 48processes the intensity information measured by the detector array 66.

[0030] Deep ultra violet reflective spectrometry (DUV) simultaneouslyprobes the sample with multiple wavelengths of ultra-violet light. DUV16 uses the same spectrometer 58 to analyze probe beam 26 as BRS 14,except that DUV 16 uses the reflective lens 33 (FIG. 2) instead offocusing lens 32. To operate DUV 16, the turret containing lenses 32/33is rotated so that reflective lens 33 is aligned in probe beam 26. Thereflective lens 33 is necessary because solid objective lenses cannotsufficiently focus the UV light onto the sample.

[0031] Broadband spectroscopic ellipsometry (BSE) is discussed inpending U.S. patent application Ser. No. 08/685,606, filed on Jul. 24,1996, which is commonly owned by the present assignee and isincorporated herein by reference. BSE (18) includes a polarizer 70,focusing mirror 72, collimating mirror 74, rotating compensator 76, andanalyzer 80. In operation, mirror 82 directs at least part of probe beam26 to polarizer 70, which creates a known polarization state for theprobe beam, preferably a linear polarization. Mirror 72 focuses the beamonto the sample surface at an oblique angle, ideally on the order of 70degrees to the normal of the sample surface. Based upon well knownellipsometric principles, the reflected beam will generally have a mixedlinear and circular polarization state after interacting with thesample, based upon the composition and thickness of the sample's film 8and substrate 6. The reflected beam is collimated by mirror 74, whichdirects the beam to the rotating compensator 76. Compensator 76introduces a relative phase delay δ (phase retardation) between a pairof mutually orthogonal polarized optical beam components. Compensator 76is rotated at an angular velocity Ω about an axis substantially parallelto the propagation direction of the beam, preferably by an electricmotor 78. Analyzer 80, preferably another linear polarizer, mixes thepolarization states incident on it. By measuring the light transmittedby analyzer 80, the polarization state of the reflected probe beam canbe determined. Mirror 84 directs the beam to spectrometer 58, whichsimultaneously measures the intensities of the different wavelengths oflight in the reflected probe beam that pass through thecompensator/analyzer combination. Processor 48 receives the output ofthe detector 66, and processes the intensity information measured by thedetector 66 as a function of wavelength and as a function of the azimuth(rotational) angle of the compensator 76 about its axis of rotation, tosolve the ellipsometric values Ψ, and Δ as described in U.S. patentapplication Ser. No. 08/685,606.

[0032] Detector/camera 86 is positioned above mirror 46, and can be usedto view reflected beams off of the sample 4 for alignment and focuspurposes.

[0033] In order to calibrate BPE 10, BPR 12, BRS 14, DUV 16, and BSE 18,the composite optical measurement system 1 includes the wavelengthstable calibration reference ellipsometer 2 used in conjunction with areference sample 4. Ellipsometer 2 includes a light source 90, polarizer92, lenses 94 and 96, rotating compensator 98, analyzer 102 and detector104.

[0034] Light source 90 produces a quasi-monochromatic probe beam 106having a known stable wavelength and stable intensity. This can be donepassively, where light source 90 generates a very stable outputwavelength which does not vary over time (i.e. varies less than 1%).Examples of passively stable light sources are a helium-neon laser, orother gas discharge laser systems. Alternately, a non-passive system canbe used as illustrated in FIG. 3 where the light source 90 includes alight generator 91 that produces light having a wavelength that is notprecisely known or stable over time, and a monochrometer 93 thatprecisely measures the wavelength of light produced by light generator91. Examples of such light generators include solid state lasers, laserdiodes, or polychromatic light sources used in conjunction with a colorfilter such as a grating. In either case, the wavelength of beam 106,which is a known constant or measured by monochrometer 93, is providedto processor 48 so that ellipsometer 2 can accurately calibrate theoptical measurement devices in system 1.

[0035] The beam 106 interacts with polarizer 92 to create a knownpolarization state. In the preferred embodiment, polarizer 92 is alinear polarizer made from a quartz Rochon prism, but in general thepolarization does not necessarily have to be linear, nor even complete.Polarizer 92 can also be made from calcite. The azimuth angle ofpolarizer 92 is oriented so that the plane of the electric vectorassociated with the linearly polarized beam exiting from the polarizer92 is at a known angle with respect to the plane of incidence (definedby the propagation direction of the beam 106 and the normal to thesurface of sample 4). The azimuth angle is preferably selected to be onthe order of 30 degrees because the sensitivity is optimized when thereflected intensities of the P and S polarized components areapproximately balanced. It should be noted that polarizer 92 can beomitted if the light source 90 emits light with the desired knownpolarization state.

[0036] The beam 106 is focused onto the sample 4 by lens 94 at anoblique angle. For calibration purposes, reference sample 4 ideallyconsists of a thin oxide layer 8 having a thickness d, formed on asilicon substrate 6. However, in general, the sample 4 can be anyappropriate substrate of known composition, including a bare siliconwafer, and silicon wafer substrates having one or more thin filmsthereon. The thickness d of the layer 8 need not be known, or beconsistent between periodic calibrations. The useful light from probebeam 106 is the light reflected by the sample 4 symmetrically to theincident beam about the normal to the sample surface. It is notedhowever that the polarization state of nonspecularly scattered radiationcan be determined by the method of the present invention as well. Thebeam 106 is ideally incident on sample 4 at an angle on the order of 70degrees to the normal of the sample surface because sensitivity tosample properties is maximized in the vicinity of the Brewster orpseudo-Brewster angle of a material. Based upon well known ellipsometricprinciples, the reflected beam will generally have a mixed linear andcircular polarization state after interacting with the sample, ascompared to the linear polarization state of the incoming beam. Lens 96collimates beam 106 after its reflection off of the sample 4.

[0037] The beam 106 then passes through the rotating compensator(retarder) 98, which introduces a relative phase delay 6 (phaseretardation) between a pair of mutually orthogonal polarized opticalbeam components. The amount of phase retardation is a function of thewavelength, the dispersion characteristics of the material used to formthe compensator, and the thickness of the compensator. Compensator 98 isrotated at an angular velocity ω about an axis substantially parallel tothe propagation direction of beam 106, preferably by an electric motor100. Compensator 98 can be any conventional wave-plate compensator, forexample those made of crystal quartz. The thickness and material of thecompensator 98 are selected such that a desired phase retardation of thebeam is induced. In the preferred embodiment, compensator 98 is abi-plate compensator constructed of two parallel plates of anisotropic(usually birefringent) material, such as quartz crystals of oppositehandedness, where the fast axes of the two plates are perpendicular toeach other and the thicknesses are nearly equal, differing only byenough to realize a net first-order retardation for the wavelengthproduced by the light source 90.

[0038] Beam 106 then interacts with analyzer 102, which serves to mixthe polarization states incident on it. In this embodiment, analyzer 102is another linear polarizer, preferably oriented at an azimuth angle of45 degrees relative to the plane of incidence. However, any opticaldevice that serves to appropriately mix the incoming polarization statescan be used as an analyzer. The analyzer 102 is preferably a quartzRochon or Wollaston prism. The rotating compensator 98 changes thepolarization state of the beam as it rotates such that the lighttransmitted by analyzer 102 is characterized by: $\begin{matrix}\begin{matrix}{{I(t)} = \quad {\left( {1/2} \right)\left\lbrack {\left. {{E_{x}}^{2}\left( {1 + {\cos^{2}\left( {\delta/2} \right)} + {{E_{y}}^{2}{\sin^{2}\left( {\delta/2} \right)}}} \right.} \right\rbrack -} \right.}} \\{\quad {{{{Im}\left( {E_{x}E_{y}^{*}} \right)}\sin \quad {{\delta sin}\left( {2\quad \omega \quad t} \right)}} +}} \\{\quad {{{{Re}\left( {E_{x}E_{y}^{*}} \right)}{\sin^{2}\left( {\delta/2} \right)}\sin \quad \left( {4\quad \omega \quad t} \right)} +}} \\{\quad {\left( {1/2} \right)\quad \left( {{E_{x}}^{2} - {E_{y}}^{2}} \right){\sin^{2}\left( {\delta/2} \right)}\cos \quad \left( {4\quad \omega \quad t} \right)}}\end{matrix} & (1)\end{matrix}$

=a₀ +b ₂sin(2ωt)+a ₄cos(4ωt)+b ₄sin(4ωt)  (2)

[0039] where E_(x) and E_(y) are the projections of the incidentelectric field vector parallel and perpendicular, respectively, to thetransmission axis of the analyzer, δ is the phase retardation of thecompensator, and ω is the angular rotational frequency of thecompensator.

[0040] For linearly polarized light reflected at non-normal incidencefrom the specular sample, we have

E_(x)=r_(p)cosP  (3a)

E_(y)=r_(s)sinP  (3b)

[0041] where P is the azimuth angle of the incident light with respectto the plane of incidence. The coefficients a₀, b₂, a₄, and b₄ can becombined in various ways to determine the complex reflectance ratio:

r _(p) /r _(s)=tanψe ^(iΔ).  (4)

[0042] It should be noted that the compensator 98 can be located eitherbetween the sample 4 and the analyzer 102 (as shown in FIG. 1), orbetween the sample 4 and the polarizer 92, with appropriate and wellknown minor changes to the equations. It should also be noted thatpolarizer 70, lenses 94/96, compensator 98 and polarizer 102 are alloptimized in their construction for the specific wavelength of lightproduced by light source 90, which maximizes the accuracy ofellipsometer 2.

[0043] Beam 106 then enters detector 104, which measures the intensityof the beam passing through the compensator/analyzer combination. Theprocessor 48 processes the intensity information measured by thedetector 104 to determine the polarization state of the light afterinteracting with the analyzer, and therefore the ellipsometricparameters of the sample. This information processing includes measuringbeam intensity as a function of the azimuth (rotational) angle of thecompensator about its axis of rotation. This measurement of intensity asa function of compensator rotational angle is effectively a measurementof the intensity of beam 106 as a function of time, since thecompensator angular velocity is usually known and a constant.

[0044] By knowing the composition of reference sample 4, and by knowingthe exact wavelength of light generated by light source 90, the opticalproperties of reference sample 4, such as film thickness d, refractiveindex and extinction coefficients, etc., can be determined byellipsometer 2. If the film is very thin, such as less than or equal toabout 20 angstroms, the thickness d can be found to first order in d/λby solving $\begin{matrix}{{\frac{\rho - \rho_{o}}{\rho_{o}} = {\frac{4\quad \pi \quad {id}\quad \cos \quad \theta}{\lambda}{\varepsilon_{s}\left( {\varepsilon_{s} - \varepsilon_{o}} \right)}\quad \frac{\left( {\varepsilon_{o} - \varepsilon_{a}} \right)}{{\varepsilon_{o}\left( {\varepsilon_{s} - \varepsilon_{a}} \right)}\quad \left( {{\varepsilon_{s}\cot^{2}\theta} - \varepsilon_{a}} \right)}}},} & (5)\end{matrix}$

[0045] where

ρ_(o)=tanψ_(o) e ^(iΔo)  (6)

[0046] $\begin{matrix}{= \frac{{\sin^{2}\theta} - {\cos \quad {\theta \left( {{\varepsilon_{s}/\varepsilon_{a}} - {\sin^{2}\theta}} \right)}^{1/2}}}{{\sin^{2}\theta} + {\cos \quad {\theta \left( {{\varepsilon_{s}/\varepsilon_{a}} - {\sin^{2}\theta}} \right)}^{1/2}}}} & (7)\end{matrix}$

[0047] which is the value of ρ=tanψe^(iΔ) for d=0. Here, λ=wavelength oflight; and ε_(s), 68 _(o) and ε_(a) are the dielectric functions of thesubstrate, thin oxide film, and ambient, respectively, and θ is theangle of incidence.

[0048] If the film thickness d is not small, then it can be obtained bysolving the equations

ρ=r _(p) /r _(s), where  (8) $\begin{matrix}{r_{p} = \frac{r_{p,{oa}} + {Zr}_{p,{so}}}{1 + {{Zr}_{p,{oa}}r_{p,{so}}}}} & (9) \\{r_{s} = \frac{r_{s,{oa}} + {Zr}_{s,{so}}}{1 + {{Zr}_{s,{oa}}r_{s,{so}}}}} & (10)\end{matrix}$

[0049] and where

Z=e^(2ik d),  (11)

ck _(o⊥) /ω=n _(o⊥)=(ε_(o) /ε _(a)−sin²θ)^(½)  (12)

[0050] $\begin{matrix}{r_{s,{so}} = \frac{n_{o\bot} - n_{s\bot}}{n_{o\bot} + n_{s\bot}}} & (13) \\{r_{s,{oa}} = \frac{n_{a\bot} - n_{o\bot}}{n_{a\bot} + n_{o\bot}}} & (14) \\{r_{p,{so}} = \frac{{\varepsilon_{s}n_{o\bot}} - {\varepsilon_{o}n_{s\bot}}}{{{\varepsilon \quad}_{s}n_{o\bot}} + {\varepsilon_{o}n_{s\bot}}}} & (15) \\{r_{p,{oa}} = \frac{{\varepsilon_{o}n_{a\bot}} - {\varepsilon_{a}n_{o\bot}}}{{{\varepsilon \quad}_{o}n_{a\bot}} + {\varepsilon_{a}n_{o\bot}}}} & (16)\end{matrix}$

[0051] and in general

n _(j⊥)=(ε_(j)−ε_(a)sin²θ)^(½)  (17)

[0052] where j is s or a. These equations generally have to be solvednumerically for d and n_(o) simultaneously, using ε_(s), ε_(a), λ, andθ, which are known.

[0053] Once the thickness d of film 8 has been determined byellipsometer 2, then the same sample 4 is probed by the other opticalmeasurement devices BPE 10, BPR 12, BRS 14, DUV 16, and BSE 18 whichmeasure various optical parameters of the sample 4. Processor 48 thencalibrates the processing variables used to analyze the results fromthese optical measurement devices so that they produce accurate results.For each of these measurement devices, there are system variables thataffect the measured data and need to be accounted for before an accuratemeasurement of other samples can be made. In the case of BPE 10, themost significant variable system parameter is the phase shift thatoccurs due to the optical elements along the BPE optical path.Environmental changes to these optical elements result in an overalldrift in the ellipsometric parameter Δ, which then translates into asample thickness drift calculated by the processor 48 from BPE 10. Usingthe measured optical parameters of BPE 10 on reference sample 4, andusing Equation 5 and the thickness of film 8 as determined fromcalibration ellipsometer 2, the processor 48 calibrates BPE 10 byderiving a phase offset which is applied to measured results from BPE 10for other samples, thereby establishing an accurate BPE measurement. ForBSE 18, multiple phase offsets are derived for multiple wavelengths inhe measured spectrum.

[0054] For the remaining measurement devices, BPR 12, BRS 14 and DUV 16,the measured reflectances can also be affected by environmental changesto the optical elements in the beam paths. Therefore, the reflectancesR_(ref) measured by BPR 12, BRS 14 and DUV 16 for the reference sample 4are used, in combination with the measurements by ellipsometer 2, tocalibrate these systems. Equations 9-17 are used to calculate theabsolute reflectances RCref of reference sample 4 from the measuredresults of ellipsometer 2. All measurements by the BPR/BRS/DUV devicesof reflectance (R_(s)) for any other sample are then scaled by processor48 using the normalizing factor in equation 18 below to result inaccurate reflectances R derived from the BPR, BRS and DUV devices:

R=R _(s)(R ^(c) _(ref) /R _(ref))  (18)

[0055] In the above described calibration techniques, all systemvariables affecting phase and intensity are determined and compensatedfor using the phase offset and reflectance normalizing factor discussedabove, thus rendering the optical measurements made by these calibratedoptical measurement devices absolute.

[0056] The above described calibration techniques are based largely uponcalibration using the derived thickness d of the thin film. However,calibration using ellipsometer 2 can be based upon any of the opticalproperties of the reference sample that are measurable or determinableby ellipsometer 2 and/or are otherwise known, whether the sample has asingle film thereon, has multiple films thereon, or even has no filmthereon (bare sample).

[0057] The advantage of the present invention is that a reference samplehaving no thin film thereon, or having thin film thereon with an unknownthickness which may even vary slowly over time, can be repeatedly usedto accurately calibrate ultra-sensitive optical measurement devices.

[0058] The output of light source 90 can also be used to calibrate thewavelength measurements made by spectrometer 58. The sample 4 can betipped, or replaced by a tipped mirror, to direct beam 106 up to mirror42 and to dispersion element 64. By knowing the exact wavelength oflight produced by light source 90, processor 48 can calibrate the outputof detector 66 by determining which pixel(s) corresponds to thatwavelength of light.

[0059] It should be noted that the calibrating ellipsometer 2 of thepresent invention is not limited to the specific rotating compensatorellipsometer configuration discussed above. The scope of the presentinvention includes any ellipsometer configuration in conjunction withthe light source 90 (having a known wavelength) that measures thepolarization state of the beam after interaction with the sample andprovides the necessary information about sample 4 for calibratingnon-contact optical measurement devices.

[0060] For example, another ellipsometric configuration is to rotatepolarizer 92 or analyzer 100 with motor 100, instead of rotating thecompensator 98. The above calculations for solving for thickness d stillapply.

[0061] In addition, null ellipsometry, which uses the same elements asellipsometer 2 of FIG. 1, can be used to determine the film thickness dfor calibration purposes. The ellipsometric information is derived byaligning the azimuthal angles of these elements until a null or minimumlevel intensity is measured by the detector 104. In the preferred nullellipsometry embodiment, polarizers 92 and 102 are linear polarizers,and compensator 98 is a quarter-wave plate. Compensator 98 is aligned sothat its fast axis is at an azimuthal angle of 45 degrees relative tothe plane of incidence of the sample 4. Polarizer 92 has a transmissionaxis that forms an azimuthal angle P relative to the plane of incidence,and polarizer 102 has a transmission axis that forms an azimuthal angleA relative to the plane of incidence. Polarizers 92 and 102 are rotatedabout beam 106 such that the light is completely extinguished(minimized) by the analyzer 102. In general, there are two polarizer92/102 orientations (P₁, A₁) and (P₂, A₂) that satisfy this conditionand extinguish the light. With the compensator inducing a 90 degreephase shift and oriented with an azimuthal angle at 45 degree relativeto the plane of incidence, we have:

P ₂ =P ₁±π  (19)

A ₂ =−A ₁  (20)

ψ=A ₁>0  (21)

[0062] (where A₁ is the condition for which A is positive).

Δ=2P ₁+π/2  (22)

[0063] which, when combined with equations 5-10, allows the processor tosolve for thickness d.

[0064] Null ellipsometry is very accurate because the results dependentirely on the measurement of mechanical angles, and are independent ofintensity. Null ellipsometry is further discussed by R. M. A. Azzam andN. M. Bashara, in Ellipsometry and Polarized Light (North-Holland,Amsterdam, 1977); and by D. E. Aspnes, in Optical Properties of Solids:New Developments, ed. B. O. Seraphin (North-Holland, Amsterdam, 1976),p. 799.

[0065] It is also conceivable to omit compensator 98 from ellipsometer2, and use motor 100 to rotate polarizer 92 or analyzer 102. Either thepolarizer 92 or the analyzer 102 is rotated so that the detector signalcan be used to accurately measure the linear polarization component ofthe reflected beam. Then, the circularly polarized component is inferredby assuming that the beam is totally polarized, and what is not linearlypolarized must be circularly polarized. Such an ellipsometer, commonlycalled a rotating-polarizer or rotating-analyzer ellipsometer, is termed“an incomplete” polarimeter, because it is insensitive to the handednessof the circularly polarized component and exhibits poor performance whenthe light being analyzed is either nearly completely linearly polarizedor possesses a depolarized component. However, using UV light fromsource 90, the substrate of materials such as silicon contribute enoughto the overall phase shift of the light interacting with the sample thataccurate results can be obtained without the use of a compensator. Insuch a case, the same formulas above can be used to derive thickness d,where the phase shift induced by the compensator is set to be zero.

[0066] It is to be understood that the present invention is not limitedto the embodiments described above and illustrated herein, butencompasses any and all variations falling within the scope of theappended claims. For example, beams 24, 26, and/or 106 can betransmitted through the sample, where the beam properties (including thebeam polarization state) of the transmitted beam are measured. Further,a second compensator can be added, where the first compensator islocated between the sample and the analyzer, and the second compensatorlocated between the sample and the light source 90, as illustrated inFIG. 4. These compensators could be static or rotating. In addition, toprovide a static or varying retardation between the polarization states,compensator 98 can be replaced by a non-rotating opto-electronic elementor photo-elastic element, such as a piezo-electric cell retarder whichare commonly used in the art to induce a sinusoidal or static phaseretardation by applying a varying or static voltage to the cell.

[0067] After the apparatus has been calibrated, it can be used to make avariety of measurements. One type of measurement of significant interestto the semiconductor industry is the characterization of multi-layerthin films on a substrate. FIG. 5 is an illustration of such a sample200. Sample 200 includes a semiconductor substrate 202 which istypically silicon but could be germanium, gallium arsenide, etc. Aplurality of thin film layers are deposited on top of the substrate. Thethickness of these layers in the illustration has been exaggerated forclarity.

[0068] As seen in the example of FIG. 5, four thin film layers 204 to210 are deposited on the stack. The most typical materials used to formthin film layers include oxides, nitrides, polysilicon, titanium andtitanium-nitride. Each of these materials have different opticalcharacteristics. As the number and variation of the thin film layersincreases, it becomes increasingly difficult to determinecharacteristics of individual layers even if multiple measurements aretaken.

[0069] In accordance with the subject invention, the referenceellipsometer of the subject system can further be used to help betteranalyze complex multi-layer stacks. Although the output from thereference ellipsometer is limited and is not particularly helpful inanalyzing individual layers in a stack, it can be used to provide a veryaccurate determination of the total optical thickness T of the stack. Asdiscussed below, the processor 48 can use the measurements obtained fromthe reference ellipsometer in combination with the other measurements toimprove the accuracy of the analysis of the individual layers.

[0070]FIG. 6 is a flow chart illustrating how the system can beconfigured to analyze multi-layer stacks. The steps shown in FIG. 6would generally occur after calibration in the manner discussed above.In addition, it should be noted that the data gathering steps are shownin sequential order in FIG. 6 for illustration purposes only. In fact,the various measurements can be made in any order. The results arestored in the processor as each measurement is completed. When all thedesired measurements are completed, then the processor can analyze thedata.

[0071] In accordance with the subject invention, the ellipsometer 2 isused to measure the test sample (step 230). In this case, the testsample 200 would be placed in the apparatus in place of the referencesample 4 shown in FIG. 1. The output from the measurement, in the formof first output signals, would be sent to the processor 48 in step 232.

[0072] As noted above, the output of the ellipsometer 2 will be used tocalculate the total optical thickness T of the layers. To the extentthat the ellipsometer is used for this purpose, it is preferable thatthe light source 90 be a laser which generates a fixed and knownwavelength. In the preferred embodiment, light source 90 is a heliumneon laser having a fixed output of 632.8 nanometers. The advantage ofthe helium neon laser is that it is low in cost, can be tightly focusedand generates a known wavelength output regardless of room temperatureor power levels.

[0073] In accordance with the subject invention, additional measurementsmust be taken in order to analyze the characteristics of individuallayers. In the preferred embodiment, the most desirable measurement willinclude a multi-wavelength measurement as shown in step 234. Thismulti-wavelength measurement may be based on either the change in phaseor magnitude of a reflected beam. As noted above, the white light source22 can be used for either type of measurement. The detector 58 canmeasure changes in magnitude of the reflected beam across a largewavelength range for either the broadband reflective spectrometer (BRS)14 or the deep ultraviolet reflective spectrometer (DUV) 16. Thedetector 58 generates output signals corresponding to a plurality ofwavelengths. Step 236 indicates the spectroscopic magnitude measurement.

[0074] Changes in phase of the beam at multiple wavelengths can beobtained from the broadband spectroscopic ellipsometer (BSE) 18. Step238 illustrates the BSE measurement. The second output signalscorresponding to the different wavelengths of either type ofmulti-wavelength measurement are sent to the processor 48 for storage(step 240). In the preferred embodiment, both magnitude and phasemeasurements are taken and sent to the processor.

[0075] Additional measurements are desirable to help more accuratelycharacterize the layers. In the preferred embodiment, these measurementsinclude those taken by the beam profile ellipsometer system (BPE) instep 242 and beam profile reflectometer system (BPR) in step 244. Theresults from these measurements are sent to the processor in step 246.

[0076] In accordance with the subject invention, the processor can usethe combination of inputs from the measurement systems to characterizethe sample. As noted above, the processor will typically include amodeling algorithm which utilizes an iterative process such as a leastsquares fitting routine to determine the characteristics of individuallayers. (See, for example, the Fanton, et. al. and Leng et. al.articles, cited above). In these types of routines, an initialcalculation of the parameters of the stack is made using Fresnelequations and a predetermined “best guess” of layer characteristics. Thecalculation produces a set of theoretical values which correspond to aset of measurement results that can be obtained using the various testsystems in the device. The set of theoretical values are then comparedto the set of measurements that were actually obtained from the varioustest systems and an evaluation is made as to the closeness or “fit”between the actual and theoretical values. A new “best guess” is thenmade as to the layer characteristics based on how much and in whatmanner the theoretical values differed from the measured values. Thealgorithm recalculates the parameters once again using the Fresnelequations and another comparison is made between the revised theoreticalvalues and the experimentally obtained measurements. This process iscontinued in an iterative fashion until the theoretical values match theactual measured values to a predetermined level of accuracy.

[0077] In accordance with the subject invention, this mathematicalmodeling is expanded to include parameters representative of the totaloptical thickness of the stack (step 250). This analysis assumes thatthe multilayers stack is actually a single layer with commoncharacteristics. The model will generate a set of additional theoreticalvalues corresponding to the measurements which should be generated bythe narrow-band, off-axis ellipsometer measurement. During the iterativeprocess, these theoretical values associated with the total opticalthickness are compared with actual measured values obtained from theoff-axis ellipsometer. The closeness of the “fit” between all of thetheoretical values (including the values associated with the totaloptical thickness) and all of the measured values is evaluated in theiterative process to generate a more accurate analysis of thecharacteristics of the individual layers in the stack.

[0078] The improvements achieved by this approach are derived from thefact that the total optical thickness of a stack can be very accuratelydetermined from measurements using an off-axis ellipsometer with a knownwavelength. For stacks ranging up to 200 angstroms thick, this type ofmeasurement can be accurate to within a single angstrom or less.

[0079] The subject invention is not limited to the particular algorithmused to derive the characteristics of the individual layers. In additionto the more conventional least square fitting routines, alternativeapproaches can be used. For example, the high level of computing powernow available permits approaches to be utilized which include geneticalgorithms. One example of the use of genetic algorithms to determinethe thickness of thin film layers can be found in “Using GeneticAlgorithms with Local Search for Thin Film Metrology,” Land, et. al.,Proceeding of the Seventh International Conference on GeneticAlgorithms, July 19-23, page 537, 1997. The only requirement of thesubject invention is that the algorithm be designed such that themeasurements from the off-axis ellipsometer be used to evaluate thetheoretical overall optical thickness of the multilayer stack and thatthis information be used to help minimize ambiguities in the analysis ofthe characteristics of the individual layers.

[0080] While the subject invention has been described with reference toa preferred embodiment, various changes and modifications could be madetherein, by one skilled in the art, without varying from the scope andspirit of the subject invention as defined by the appended claims.

What is claimed is:
 1. A method of analyzing a sample having a multiplelayer thin film stack thereon comprising the steps of: measuring thesample using an off-axis ellipsometer which includes a stable narrowband wavelength source and generating first output signals; measuringthe response of the sample to reflected light from a broad bandwavelength source and generating a plurality of second output signalscorresponding to different wavelengths; and determining thecharacteristics of the individual layers on the sample based on thefirst and second output signals using an algorithm wherein the firstoutput signals are used to provide an accurate measure of the overalloptical thickness of the stack in order to improve the accuracy of theanalysis of the individual layers.
 2. A method as recited in claim 1wherein the narrow band wavelength source is defined by a gas dischargelaser.
 3. A method as recited in claim 1 wherein the narrow bandwavelength source is defined by a laser diode.
 4. A method as recited inclaim 1 wherein the narrow band wavelength source is defined by a solidstate laser.
 5. A method as recited in claim 1 wherein said narrow bandwavelength source is linearly polarized prior to striking the sample andwherein the polarization change on reflection is monitored using arotating compensator and analyzer.
 6. A method as recited in claim 1wherein the step of measuring with a broad band wavelength sourceincludes the step of illuminating the sample with multiple wavelengthsof light simultaneously.
 7. A method as recited in claim 1 wherein thestep of measuring with a broad band wavelength source includes the stepof illuminating the sample with multiple wavelengths of lightsequentially.
 8. A method as recited in claim 1 wherein the step ofmeasuring the sample response to a broad band wavelength source includesanalyzing the change in polarization state of the light induced byreflection off the surface of the sample.
 9. A method as recited inclaim 1 wherein the step of measuring the sample response to a broadband wavelength source includes analyzing the change in magnitude of thelight induced by reflection off the surface of the sample.
 10. A methodas recited in claim 1 wherein the step of measuring the sample responseto a broad band wavelength source includes light spanning a wavelengthrange of 200 nm to 800 nm.
 11. A method as recited in claim 1 furtherincluding the step of measuring the response of the sample to reflectedlight at one or more wavelengths and at a plurality of different anglesof incidence and generating third output signals and using the thirdoutput signals to further characterize the individual layers on thesample.
 12. A method of determining the characteristics of individuallayers in a thin film stack formed on a sample comprising the steps of:generating a first probe beam defined by quasi-monochromatic light of aknown wavelength; directing the first probe beam to reflect off thesurface of the sample at a non-normal angle of incidence; analyzing thechange in polarization state of the first probe beam induced by theinteraction with the sample and generating first output signals inresponse thereto; generating a second probe beam from a broad bandwavelength source; directing said second probe beam to reflect off thesurface of the sample; monitoring the second probe beam after reflectionfrom the sample and determining either a phase or magnitude thereof at aplurality of wavelengths and generating a plurality of second outputsignals corresponding thereto; and determining the characteristics ofthe individual layers on the sample based on the first and second outputsignals using an algorithm wherein the first output signals are used toprovide an accurate measure of the overall optical thickness of thestack in order to improve the accuracy of the analysis of the individuallayers.
 13. A method as recited in claim 12 wherein said second probebeam is directed to the surface of the sample in a manner such thatmultiple wavelengths of light strike the surface simultaneously.
 14. Amethod as recited in claim 12 wherein said second probe beam is directedto the surface of the sample in a manner such that multiple wavelengthsof light strike the surface of the sample sequentially.
 15. A method asrecited in claim 12 wherein said step of monitoring the second probebeam includes analyzing the change in polarization state of the beaminduced by reflection off the surface of the sample.
 16. A method asrecited in claim 12 wherein said step of monitoring the second probebeam includes analyzing the change in magnitude of the beam induced byreflection off the surface of the sample
 17. A method as recited inclaim 12 wherein the first probe beam is generated by a gas dischargelaser.
 18. A method as recited in claim 12 wherein the first probe beamis generated by a laser diode.
 19. A method as recited in claim 12wherein the first probe beam is generated by a solid state laser.
 20. Amethod as recited in claim 12 wherein the light in said first probe beamis linearly polarized prior to striking the sample and wherein thepolarization change on reflection is monitored using a rotatingcompensator and analyzer.
 21. A method as recited in claim 12 whereinthe broad band wavelength source includes light spanning a wavelengthrange of 200 nm to 800 nm.
 22. A method as recited in claim 12 furtherincluding the step of measuring the response of the sample to reflectedlight at one or more wavelengths and at a plurality of different anglesof incidence and generating third output signals and using the thirdoutput signals to further characterize the individual layers on thesample.
 23. An apparatus for characterizing thin film layers in a stackformed on a sample comprising: an off-axis ellipsometer, saidellipsometer having a quasi-monochromatic source for generating a firstprobe beam of a known wavelength, said ellipsometer for measuring thechange in polarization state of the first probe beam after reflectionfrom the sample and generating first output signals correspondingthereto; a broad band light source for generating a second probe beam; adetector system for monitoring either the phase or magnitude changes ofthe second probe beam after interacting with the sample and generating aplurality of second output signals corresponding to a plurality ofdifferent wavelengths; and processor for determining the characteristicsof the individual layers on the sample based on the first and secondoutput signals, said processor using an algorithm wherein the firstoutput signals are used to provide an accurate measure of the overalloptical thickness of the stack in order to improve the accuracy of theanalysis of the individual layers.
 24. An apparatus as recited in claim23 wherein said second probe beam is directed to the surface of thesample in a manner such that multiple wavelengths of light strike thesurface simultaneously.
 25. An apparatus as recited in claim 23 whereinsaid second probe is directed to the surface of the sample in a mannersuch that multiple wavelengths of light strike the surface of the samplesequentially
 26. An apparatus as recited in claim 23 wherein thedetector system analyzes the change in polarization state of the secondprobe beam induced by reflection off the surface of the sample.
 27. Anapparatus as recited in claim 23 wherein the detector system analyzesthe change in magnitude of the second probe beam induced by reflectionoff the surface of the sample.
 28. An apparatus as recited in claim 23wherein the detector system analyzes both the change in polarizationstate of the second probe beam induced by reflection off the surface ofthe sample and the change in magnitude of the second probe beam inducedby reflection off the surface of the sample and wherein the processoruses the output signals generated by both measurements to furthercharacterize the sample.
 29. An apparatus as recited in claim 23 whereinthe quasi-monochromatic source is defined by a gas discharge laser. 30.An apparatus as recited in claim 23 wherein the quasi-monochromaticsource is defined by a helium-neon laser.
 31. An apparatus as recited inclaim 23 wherein the quasi-monochromatic source is defined by a solidstate laser.
 32. An apparatus as recited in claim 23 wherein thequasi-monochromatic source is defined by a laser diode.
 33. An apparatusas recited in claim 23 wherein the light in said first probe beam islinearly polarized prior to striking the sample and wherein thepolarization change on reflection is monitored using a rotatingcompensator and analyzer.
 34. An apparatus as recited in claim 23wherein the broad band wavelength source includes light spanning awavelength range of 200 nm to 800 nm.
 35. An apparatus as recited inclaim 23 wherein the detector system further includes the step ofmeasuring the response of the sample to reflected light at one or morewavelengths and at a plurality of different angles of incidence andgenerating third output signals and using the third output signals tofurther characterize the individual layers on the sample.
 36. Anapparatus as recited in claim 35 wherein said light which is measured ata plurality of different angles of incidence is generated by a laser.37. The reference ellipsometer of claim 23 , wherein thequasi-monochromatic source produces light having a stable knownwavelength to within 1 percent.